On the evening of the 25 March, the Barcelona Raman LIDAR Pathfinder installed on the CTA-North site at the Roque de los Muchachos Observatory on La Palma (Spain) achieved first light.

In spite of windy conditions, the LIDAR team, after a laborious laser alignment, was able to operate the instrument smoothly and, as scheduled, produce data. Built in Barcelona and installed at the CTA-North site inside the LST-1 construction area in mid-February 2021, the LIDAR will play a fundamental role in characterizing the atmospheric properties and calibrating the data acquired by the CTA telescopes. It will be tested for about a year, then returned back to Barcelona for updates and improvements based on the data collected. A final instrument based on the pathfinder will be installed at CTA-North. A similar instrument, built by the University of Montpellier, France, will be installed at the CTA-South site. Now that the team has achieved first light, regular data collection for commissioning of the instrument will follow.

“This is a major milestone for our project and an important step toward demonstrating that the LIDAR will be incremental to reducing the systematic uncertainties of CTA at an unprecedented level in our field,” said the project’s principal investigators Manel Martinez and Markus Gaug.

A LIDAR is a remote sensing instrument used to measure the vertical profiles of aerosol and water vapor within the atmosphere. It works similarly to a RADAR (LIDAR stands for Light Detection And Ranging) but at much shorter visible or ultraviolet wavelengths rather than radio. It features a powerful laser and a telescope: the laser shoots a series of meter-long pulses into the atmosphere, each with the power of several megawatts, while the telescope collects the radiation backscattered by various atmospheric components.

From the time required for light to travel back to the telescope, it is possible to derive the exact content of aerosol and water vapor at each altitude. Using this information, vertical profiles can be created for both components to determine the so-called “extinction coefficient” –  a measure of how radiation coming from sky sources is altered as it passes through the atmosphere, a parameter of paramount importance for calibrating CTA telescopes.

The Barcelona Raman LIDAR pathfinder for CTA-North is a joint project between CTA members from IFAE-BIST (Institute of High Energy Physics – Barcelona Institute of Science and Technology), UAB & IEEC-CERES (Autonomous University of Barcelona & Institute of Space Studies of Catalonia-Center of Space Studies and Research), Center of Astrophysics and Cosmology of the University of Nova Gorica and Department of Physics and Astronomy of the University of Padova. Paolo Calisse, the CTA-North Site Manager, serves as the on-site project manager for the instrument.